The present invention relates generally to signal amplification, and more particularly to a dual gate amplifier having low noise characteristics.
A low noise amplifier (LNA) is utilized in various aspects of wireless communications, including wireless LANs, cellular communications, and satellite communications. A LNA is often utilized in an input stage of a signal receiving apparatus or receiver. For example, the LNA amplifies a received RF (Radio Frequency) input signal so that an amplified RF signal has a desired gain and noise figure.
Two main concerns associated with the design of amplifiers utilized for low noise RF amplification include: (1) the minimization of noise added to the signal by the amplifier; and (2) achieving maximum power transfer between a source producing the electrical signal and the amplifier. A figure of merit for noise added by the amplifier is the ratio of the signal-to-noise ratio at the input to the signal-to-noise ratio at the output of the amplifier. This ratio is commonly referred to as the Noise Factor (F) of the amplifier, and is used to calculate the Noise Figure (NF) of the amplifier according to the formula NF=10 log10 (F), where F=(S/N)IN/(S/N)OUT. A measurement from a noise figure meter also can be employed to ascertain noise associated with operation of the amplifier.
In bipolar technology, a popular low noise amplifier topology is the Darlington feedback amplifier. The Darlington feedback amplifier includes a Darlington-connected pair of bipolar transistors, which tend to provide increased input impedance relative to a single bipolar transistor. The conventional Darlington feedback amplifier typically employs resistive feedback and is operative to achieve broad frequency bandwidths. As a result, such an amplifier can achieve reasonably good input and output matching. Broadband amplifiers of this type tend to employ feedback paths that introduce additional unwanted noise into the amplifier output. The conventional wide bandwidth amplifiers, such as the Darlington-connected feedback type are generally achieved at the expense of realizing higher amounts of noise. While the Darlington pair resistive feedback amplifier is traditionally used for medium to high gain applications, the current gain and base resistance of the active device tend to limit the noise figure. These and other conventional amplifier approaches usually fail to optimize source impedance matching for microwave devices, which also can result in added noise.
Designing LNA""s is a trade-off between noise figure, bandwidth, VSWR, gain and stability of the amplifier. As a result, a compromise often must be reached between the end goals, which can be conflicting at times.
The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended to neither identify key or critical elements of the invention nor delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.
The present invention relates generally to a low noise amplifier that includes a dual gate transistor, such as a dual gate field effect transistor (FET) or high electron mobility transistor (HEMT) device. The amplifier includes a feedback network having a resistive component and a DC blocking component, such as a capacitor, coupled between an input and output of the amplifier.
According to one aspect of the present invention, the DC blocking component is located at an output side of the feedback network. The resistive component can be connected at an input side of the feedback network in series with the DC blocking component.
In order to operate an amplifier configured in accordance with an aspect of the present invention, a first bias network can be provided to feed a first bias voltage to a node of the feedback network for biasing a first gate of the dual gate transistor device, which is coupled to the amplifier input. A second bias network is coupled to bias another gate of the dual gate transistor device. The RF input is provided to the first gate and is amplified by the amplifier.
Such an arrangement enables improved performance, including low noise figure and a high gain, compared to conventional amplifiers designed to achieve similar gain. Additionally, an amplifier can be implemented according to an aspect of the present invention as a single stage amplifier on a single chip, occupying less real estate relative to conventional amplifier designs.
Another aspect of the present invention provides a method for implementing a low noise amplifier. The method includes providing a dual gate transistor device connected between an input and an output of the amplifier. Resistive feedback is provided between the output and a first gate of the transistor device and electrically connected with the input of the amplifier to provide the amplifier with a desired gain. The feedback also includes a DC blocking component in series with a resistive part of the resistive feedback. To operate the amplifier, the first gate of the transistor device is biased with a first DC voltage, such as through a node associated with the resistive feedback. A second gate of the transistor device is biased with a second DC voltage that is greater than the first voltage. As a result, a low noise figure (e.g., less than about 1 dB) can be achieved for the amplifier over a desired frequency range.
To the accomplishment of the foregoing and related ends, certain illustrative aspects of the invention are described herein in connection with the following description and the annexed drawings. These aspects are indicative, however, of but a few of the various ways in which the principles of the invention may be employed and the present invention is intended to include all such aspects and their equivalents. Other advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.